This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-235555, filed Aug. 3, 2000; and No. 2000-277518, filed Sep. 13, 2000, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image reading apparatus, particularly, to an image reading apparatus in which a target object to be detected is brought into contact with a photosensor array prepared by arranging a plurality of photosensors in a matrix so as to read an image pattern of the object.
2. Description of the Related Art
Known is a reading apparatus of a two dimensional image for reading, for example, fine irregularities of a printed material, a photograph, or a fingerprint, which is constructed such that a target object to be detected is disposed on or brought into contact with a detecting surface mounted to a photosensor array prepared by arranging a plurality of photoelectric converting elements in a matrix so as to read the two dimensional image of the target object.
In the two dimensional image reading apparatus of the construction that the target object is brought into direct contact with the detecting surface, there is a construction that the distance between the target object and the photoelectric converting element is short. In this case, the static electricity charged in the target object to be detected causes a driving circuit for driving the photoelectric converting elements to perform a malfunction or to be broken via the wiring of the photoelectric converting element, with the result that any photoelectric converting element itself tends to perform malfunction or to be broken.
In the conventional two dimensional image reading apparatus of the construction described above, a specific withstanding design of the means for releasing sufficiently and without fail the static electricity charged in the target object has not yet been established. Thus, the withstanding to the static electricity has not yet been achieved sufficiently. Such being the situation, it is impossible to prevent sufficiently the malfunction of the reading and the breakage of the two dimensional image reading apparatus caused by the static electricity.
Under the circumstances, it is of high importance to establish the specific construction and the forming conditions of the means capable of releasing appropriately the static electricity charged in the target object to be detected so as to prevent effectively the malfunction of reading, the breakage, etc. of the two dimensional image reading apparatus.
An object of the present invention is to provide a two dimensional image reading apparatus capable of releasing appropriately a static electricity charged in a target object to be detected, which is disposed on a photosensor device, so as to markedly suppress the malfunction of reading and the breakage of the device.
According to a first aspect of the present invention, there is provided an image reading apparatus for reading an image of a target object, comprising a plurality of sensors arranged on one side of a substrate, an insulating film formed to cover the plurality of sensors, and a conductive layer formed on the insulating film for releasing the voltage charged in the target object, the conductive layer having a sheet resistance of 50 xcexa9/xe2x96xa1 or less.
Where the target object is charged with a static electricity, the static electricity is released from the conductive layer having a low sheet resistance when the target object is brought into contact with the conductive layer so as to prevent the image reading apparatus from being broken by the static electricity.
Where the sensor may includes photosensors, it is desirable for the conductive layer to be formed of a conductive material, which transmits light. Particularly, it is desirable for the conductive layer to be formed of an ITO, i.e., a material based on indium.tin oxide. It should be noted, however, that attenuation caused by absorption and scattering of light also takes place within the ITO layer. It follows that, if the ITO layer is excessively thick, the reading sensitivity characteristics of the image pattern of the target object is deteriorated. Also, a long time is required for forming an ITO film having an excessively large thickness, leading to a low productivity. Under the circumstances, it is reasonable to form the conductive layer by using an ITO having a resistivity not higher than 2xc3x9710xe2x88x923 xcexa9xc2x7cm or less and a refractive index falling within a range of between about 2.0 and about 2.2, and it is desirable for the ITO film to have a thickness falling within a range of between about 50 nm and about 200 nm so as to set the sheet resistance of the ITO film at about 15 to 50 xcexa9/xe2x96xa1. Where the highest priority is put on the productivity, it is reasonable for the ITO film to have a thickness falling within a range of between about 50 nm and 100 nm so as to set the sheet resistance at 30 to 50 xcexa9/xe2x96xa1. On the other hand, where the highest priority is put on the charge releasing properties, it is desirable to set the thickness of the ITO film to fall within a range of between about 150 nm and 200 nm so as to set the sheet resistance at 15 to 20 xcexa9/xe2x96xa1.
According to a second aspect of the present invention, there is provided an image reading apparatus for reading the image of a target object to be detected, comprising (a) a plurality of photosensors arranged on one surface of a substrate, each of the photosensors including a semiconductor layer having an incident effective region on which an excited light is incident, a source electrode and a drain electrode formed on both edge sides of the semiconductor layer, a first gate insulating film formed below the semiconductor layer, a first gate electrode formed under the first gate insulating film, a second gate insulating film formed above the semiconductor layer, and a second gate electrode formed above the second gate insulating film, (b) an insulating film formed to cover the plural sensors, (c) a conductive layer formed on the insulating layer for releasing the voltage charged in the target object, the conductive layer having a sheet resistance not higher than 50 xcexa9/xe2x96xa1, (d) a drain driver connected the drain electrode of the plural photosensors, (e) a first gate driver connected to the first gate electrode of the plural photosensors, and (f) a second gate driver connected to the second gate electrode of the plural photosensors.
In order to prevent the light emitted from a back light arranged below from being incident directly on the semiconductor layer, the first gate electrode may be formed of a metal opaque to or reflecting the excited light. On the other hand, the second gate electrode may be formed of a transparent oxide conductor such as an ITO in order to permit the light emitted from the back light and incident on the target object to be detected and the light reflected from the target object to be transmitted through the second gate electrode. In general, since the metal has a resistivity lower than that of a transparent oxide conductor such as an ITO and, thus, is a good conductor, the metal tends to propagate easily the static electricity. Therefore, the static electricity tends to be concentrated more easily on the first gate driver than on the second gate driver so as to break the first gate driver. In the present invention, however, a conductive layer having a low sheet resistance is arranged so as to suppress the propagation of the static electricity to the driver so as to improve the withstand voltage.
According to a third aspect of the present invention, there is provided an image reading apparatus for reading the image of a target object to be detected, comprising a plurality of sensors arranged on one side of a substrate, an insulating film formed to cover the plural sensors, a conductive layer formed on the insulating film for releasing the voltage charged in the target object, the conductive layer having a sheet resistance not higher than 50 xcexa9/xe2x96xa1, and an impact alleviating layer formed on the conductive layer.
Since a semiconductor having a resistance higher than that of the conductive layer for releasing the static electricity or an insulator is used for forming the impact alleviating layer, the image reading apparatus is allowed to achieve a high withstand voltage. In addition, where the target object is a part of the charged human body, it is possible to alleviate the unpleasantness felt by the human being when the electric charge is released.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.